Saturday, August 16, 2014

Figure 1. The hindlimb and abdomen of Anolis electrum (UCMP 68496), as revealed by high-resolution X-ray computed tomography (HRXCT; A,) and light microscopy (B). The specimen mainly comprises an air-filled void in the amber that outlines the right hindlimb, left hindtoe IV, and part of the abdomen. (A) Skeleton and air-filled voids, in ventral view, are rendered opaque: the skeleton and mineralized skin are false-coloured white, the skin is false-coloured green, and an ant also preserved as an air-filled void is false-coloured brown. A yolk sac scar is clearly visible on the ventral side of the abdomen. The isolated left hindtoe IV lies on the ventral surface of the limb. (B) The limb and abdomen are clearly visible through the amber.

Anoles are well-known examples of adaptive radiation and convergent evolution. Their phylogenetic relationships have been intensely studied, but their fossil record remains fairly poor, limiting our understanding of their evolutionary history. We present new data on Anolis electrum Lazell, 1965, the first discovered fossil anole and sole vertebrate described from Mexican amber, using X-ray computed tomography. We inferred the phylogenetic relationships of A. electrum and comment on its use in estimating the age of Anolis origins, which has significant relevance in explaining the presence of anoles on Caribbean islands. Anolis electrum is represented by two pieces of amber containing parts of the same individual. Partial squamation and skeleton details are well preserved, although only ten characters commonly used in phylogenetic analyses could be scored. The lack of informative characters resulted in A. electrum being inferred in 14 different places within four recognized subclades – Dactyloa, cristatellus series, darlingtoni series, and Norops – one of which corresponds to previously suggested close relationships. Results fail to support a suggested age estimation of 130 Myr for Anolis; consequently, the hypothesis of overwater dispersal as the explanation for the occurrence of anoles on Caribbean islands remains the most robust hypothesis.

Figure 1. The hindlimb and abdomen of Anolis electrum (UCMP 68496), as revealed by high-resolution X-ray computed tomography (HRXCT; A, C, D) and light microscopy (B). The specimen mainly comprises an air-filled void in the amber that outlines the right hindlimb, left hindtoe IV, and part of the abdomen. (A) Skeleton and air-filled voids, in ventral view, are rendered opaque: the skeleton and mineralized skin are false-coloured white, the skin is false-coloured green, and an ant also preserved as an air-filled void is false-coloured brown. A yolk sac scar is clearly visible on the ventral side of the abdomen. The isolated left hindtoe IV lies on the ventral surface of the limb. (B) The limb and abdomen are clearly visible through the amber. (C) Close-up of the ventral view of the right foot and ant, showing details of the toepad lamellae. (D) Close-up of the dorsolateral view of the right hindfoot (excluding the ant) and the isolated left hindtoe IV, showing details of the limb and supradigital scales.